(1) Field of the Invention
The present invention relates to a coating process for a fatigue critical component and to a part formed thereby.
(2) Prior Art
The technology of duplex thermal spray coatings has been used for years to build up worn parts used in engines, propellers, and other applications where greater than 0.010 inches of build up is required, or in situations where a bond coat is required because the desired topcoat will not bond properly to the substrate. Tests have been conducted to identify failure modes of fatigue sensitive parts used in highly loaded applications and on which very hard wear resistant coatings are applied. Structural aluminum and titanium alloys have been found to be very sensitive to these hard coatings while steel alloys are somewhat less sensitive. These tests suggest that the high bond and cohesive strength of coatings like tungsten carbide and other cermets allow the coating to behave like the substrate. These coatings resist strain and have a modulus of elasticity equal to or greater than steel, but are brittle materials like ceramics. When a crack forms in a coating of this integrity, that crack can act just like a crack in the substrate and propagate as the theories of fracture mechanics dictate. FIGS. 1-3 show the typical crack propagation from a hard coating 10 into the softer, lower modulus structural substrate 12. As shown in FIG. 1, the crack 14 initiates in the hard, high modulus coating due to fatigue or overload. As shown in FIG. 2, the crack 14 propagates through the coating 10 and directly into the substrate 12. FIG. 3 illustrates a crack 14 extending from a tungsten carbide—17 wt % cobalt coating into a substrate formed from aluminum alloy 7075-T73.
This problem occurs in all structural materials with lower strain threshold coatings (coatings which crack with a relatively low static strain applied), but often can be avoided with very high strain threshold coating materials on steel because the modulus of elasticity of steel is so high that very high substrate stresses are required in order to generate cracks. Aluminum and titanium are still susceptible to fatigue with high strain threshold coatings due to the low modulus of elasticity of the substrate, and in the case of aluminum, the high coefficient of thermal expansion (CTE). The CTE plays a role in parts that see elevated temperatures because the CTE of most wear resistant coatings are very low. This forces a strain in the coating just due to thermal cycling, which may cause the coating to crack.